1. Field of Invention
The present invention relates to a non-volatile memory device. More particularly, the present invention relates to a method of erasing non-volatile memory data.
2. Description of Related Art
Flash memory is a type of memory that retains data even after power to the memory is cut off. Furthermore, data can be transferred into, read out or erased from a flash memory device multiples of times. Due to all these advantages, flash memory has become one of the most widely adopted non-volatile memories inside personal computers and electronic equipment.
A typical flash memory device has a floating gate and a control gate fabricated using doped polysilicon. The control gate is positioned directly on top of the floating gate but are separated from each other by a dielectric layer. The floating gate is separated from an underlying substrate by a tunnel oxide layer (in the so-called stacked gate flash memory).
To write data into the flash memory, a bias voltage is applied to the control gate and the source/drain regions such that electrons are injected into the floating gate.
On the other had, to read data from the flash memory, a working voltage is applied to the control gate so that the charging state of the floating gate determines the opening or closing of the channel underneath. According to the opening or closing of the channel, a read-out value of “1” or “0” is registered. To erase data from the flash memory, relative potential between the substrate, the drain (source) region or control gate is raised. Through the tunneling effect, electrons penetrates through the tunnel oxide layer into the substrate or drain (source) terminal (the substrate erase or drain (source) side erase) or penetrates through the dielectric layer into the control gate.
Flash memory generally uses channel hot-electron (CHE) injection mode to carry out programming and uses Fowler-Nordheim (F-N) tunneling mode to erase by freeing the trapped electrons inside the floating gate to the channel through the tunnel oxide layer.
However, when the F-N tunneling mode is used to erase data from the flash memory, the quantity of electrons expelled from the floating gate is hard to control. The floating gate may be positively charged if too many electrons are expelled from the floating gate. This phenomenon is often called over-erase. When over-erase occurs, there will be a wider distribution of threshold voltage and an increase in bit line leakage current. Furthermore, if over-erase is really serious, the channel layer underneath the control gate may conduct even without the application of any working voltage leading to data read-out error. To minimize the effect due to over-erasure, extra software programming and over-erase repair must be carried out. However, this will lead to an increase in erasing time as well as an expansion of circuit regions.